Method of manufacturing a microwave attenuator for travelling wave tube



METHOD OF MANUFACTURING A MICROWAVE ATTENUATOR j Marqh'31, 1959 J. PELLE FOR TRAVELLING WAVE TUBE Filed May 4 1954 l .l|ll

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' ATTORNEY United States Patent METHOD or MANUFACTURING" A MICROWAVE ATTENUATOR FOR TRAVELLING WAVE TUBE James S. Pelle, New York, N .Y., assignorto Sperry. Rand Corporation, a corporation of Delaware Application May 4, 1954, Serial No; 427,439 5 Claims. Cl. 117-226) This invention relates to microwave attenuators, and more particularly, is concerned with the method of making attenuatingsupportrods for the helix transmission line of a high-power travelling wave tube.

It has been common practice heretofore'to improve the performance of travelling wave tubesby attenuating in some manner reflections along the helix of the travelling wave tube, such as caused by any mismatch at the terminations of the helix that may occur a't certain'f'requencies where the tube is operated as a broad band device. See for example Patent No. 2,575,383 by L. M. Field. Because it is desirable that the attenuating material be placed close to the helix where the electricfields are greatest, it has been the general practiceto coat nonconductive rods extending along. the length of the helix with electrically resistive-material, the coated rods serving the dual purpose ofsupporting the helix along its length and attenuating the electric field surrounding the helix. Usually the rods are of a material such as glass or ceramic and the coating is colloidal graphite of particular thickness and distribution along. the rods for controlling the attenuation properties thereof. An alternative to the colloidal graphite coating has been theme of a vaporized metal film, the resistance being controlled by the density of the film.

While these two types of surface attenuators have met with a limited degree of success, they have not generally proved satisfactory in high-power travelling wave tube amplifiers. The attenuating properties of the graphite coating depends on-the intimacy of contact between individual particles of graphite in the coating. In highpower tubes, where the attenuator is called upon to'dissipate considerable power, the resultant heatingandcooling of the support rods and associated graphite coating results in agradual physical displacement or'movement of the graphite particles, causing them to flake ofi'the rod or to change their position along the rod.

Experiments with various binders to stabilize and hold the graphite particles together without insulating them from each other have not proved successful for one or more of the reasons that such binders adversely affect the attenuation properties, are not stable at elevated temperatures, or have vapor pressures which are too high for use within a vacuum system.

The metal film attenuators have proved disadvantageous for high-power applications for the reasons that the vaporizing of the coating on the surface of the rods is too critical and elaborate" to I obtain easyt reproducibility. Because of thethinness Oflhefllmplt is easily scratched or otherwise damaged. Furthermore, any metal film which can be readily vaporized to provide such a coating is not stable when raised to temperatures encountered in tubes at high-power operation. The metal, having a high surface tension, tends to collect in globules when subjected to repeated heating to elevated temperatures, so that the attenuating propertiesof thefilm are destroyed with use. Even far below melting temperatures, such thin metal layers difiuse from high power attenuation regions 2,880,120 Patented Mar. 31, 1959 ice 2 (high temperatures) to low power regions (low temperatures), and thus the attenuation is destroyed.

It is the general object of this invention to avoid and overcome the foregoing and other difliculties in and objectionsto the prior art practices by] the provision of an improved microwave attenuator that is capable of'opcrating at the'elevated temperatures associated with large power dissipation, that is further characterized byj'its stability and long operating life, and which can be made without critical or elaborate techniques.

Another object of this invention is to provide an attenuating material which can be used in the formof elongated rods for serving the dual purpose of supporting 'the helix of a travelling wave'tube and providing the'requir'ed attenuation for improved operation of the tube.

Another object of this invention is the provision of an attenuator in which the amount of attenuation per unit length can be readily controlled.

Another object of this invention is to provide an attenuator in which microwave attenuation takes place throughout the volume of the attenuator rather than only at the surface, so that the attenuating. properties are not aifected by marring or scratching or other injury occurring at the surface of the attenuator.

These and other objects of the invention which will become apparent as the description proceeds are achieved by providing an attenuator consisting of a material produced in the following manner. Aporous ceramicrod of proper length and diameter to serve as a'helix" sup.- port rod in a travelling wave tube is soaked in a solution of sucrose and water. The rod is thendried to remove the excess water and heatedfor fifteen minutes at 400 C. in an inert atmosphere or inits own vapors of decomposition, thus changing the sucrose to colloidal carbon deposited in the pores of the ceramic rod; The'rod is then fired in a vacuum for a proper period of time at an elevated temperature of 1000 C., in which process colloidal carbon is converted to graphite. The amount of sucrose material and the degree and time of firing in converting the carbon to graphite determine the resistance and attenuating properties of the rod.

For a better understanding of the invention, reference should be had to the accompanying drawing, wherein:

Fig. 1 is an elevational view, partly in section, of a travelling wave tube embodying the present invention;

Fig. 2 is a fragmentary cross-sectional view taken on the line 22of Fig. 1

Fig. 3 is an enlarged view of'the attenuating rods and associated supporting structure of the tube illustrated in Fig. 1'; and

Fig. 4 is a flow diagram illustrating'the steps in the preparation of the attenuator rods of the present invention.

Referring to Fig. 1, the numeral 10 indicates generally a travelling wave tube of a type described'in detail in the copending application Serial No. 426,323, filed April'29, 1954.

Thetravelling wave tube includes an outer shell 10 of conductive non-magnetic material with an electron gun asserhbly 14 positioned at one end and a collector 16 positioned at the opposite end. Extending axially through the tube is a helical conductor 18 supported along its length by a plurality of rods 20. Sections of coaxial transmission lines, indicated generally at 22 and 24, couple the RF. signal amplified by the tube respectively into and out of the helix transmission line 18.

A-horn member 26 is positioned within the tube adjacent the electron gun assembly 14. A central flaring opening 28 coaxial with the helix 18 receives the end of the helix. The support rods 20 are received in bores3j0 in the horn member, the rods being supported and accurately positioned thereby. The coaxial line 22 is coupled to the end of the helix within the flaring opening 28. A second horn member 32 is similarly positioned at the opposite end of the helix 18 and supports the rods 20. Fig. 3 shows the assembly of the rods 20 and supporting members 26 and 32 at each end.

As best shown in the sectional view of Fig. 2, the attenuating rods 20, which are made in the manner of the present invention, are in contact with the helix around the outer periphery thereof. Thus the rods are in a position to attenuate fields external to the helix and at the same time give mechanical support to the helix.

In accordance with the present invention, the attenuating rods 20 are prepared in the following manner, as indicated in the flow diagram of Fig. 4. A porous ceramic rod of such material as aluminum silicate or magnesium silicate of proper length and shape is used. An example of a suitable material is AI-222, American Lava Corporation of Tennessee. The rod is soaked in a 40% solution by weight of sucrose and water and dried. However, the solution may be brushed on so as to limit the portion of the rod in which the solution is applied. Where soaking is used, it may be desirable to coat portions of the rod with parafiin in the regions where it is desired not to have attenuation. Thus the attenuating properties of the rod may be confined to the central region of the rods, as indicated at 34 in Fig. 3. While a 40% solution is recommended for maximum effect, thinner solutions may be used where less attenuation is desired.

While sucrose has been found highly satisfactory, other polyhydroxyl compounds may be employed providing they are not readily vaporized before decomposition to carbon sets in. All of the sugars, for example, glucose, lactose, dextrin and other polyhydroxyl compounds are satisfactory.

After the rod is dried to remove the solvent, it is heated for at least fifteen minutes at 400 C. This heating is done either in an inert atmosphere or in a very confined space where the available oxygen is limited. As a result of the heating, the sucrose is decomposed driving off the volatile products and leaving behind a precipitate of colloidal carbon. The process of soaking and heating may be repeated two or three additional times as may be required to precipitate the desired amount of carbon in the pores of the ceramic.

After precipitating the colloidal carbon, the ceramic rod is then brought to proper resistance range by firing in an oxygen free atmosphere such as a vacuum or dry H atmosphere for at least ten minutes at 1000 C. The firing process reduces the colloidal carbon to graphite.

'The ultimate resistance of the rod per unit length may be controlled by the length of time and the temperature at which the firing of the rod takes place.

From the above description it will be seen that the various objects of the invention have been achieved by the provision of an improved microwave attenuator. Since the attenuator may be operated at high temperatures for a sustained period, it is particularly suited for high-power applications. Although the form of the attenuator has been described as a rod for use in a travelling wave tube, it will be recognized that the attenuating material produced by the above-described process may be adapted for other uses, for example, as a non-reflecting dissipative load for a microwave transmission line and similar applications. The attenuator is particularly suitable for operation within a vacuum since none of the materials involved in the final product have a high vapor pressure. Where the attenuator is to be operated at red heat, it should be used in an inert atmosphere or in a vacuum to prevent oxidation of the carbon.

vHowever, at lower temperatures, up to 400 C., such precautions need not be taken and the attenuator is highly stable even when operated in air.

Since many changes could be made in the above con- 4 struction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The method of manufacturing a microwave attenuator and support for the helix of a travelling wave tube, comprising the steps of: impregnating part of a porous ceramic rod with a solution of an organic compound, heating said rod at a low temperature for gradual decomposition of said organic compound into a carbon containing material, and firing said rod at an elevated temperature well above said low temperature for converting said material to graphite having desired microwave loss properties.

2. The method of manufacturing a microwave attenuator and support for the helix of a travelling wave tube, comprising the steps of: impregnating part of a porous ceramic rod with a solution of an organic compound, drying said rod, heating said rod in an inert atmosphere at a low temperature for gradual decomposition of said organic compound into a carbon containing material, and firing said rod in an oxygen free atmosphere at an elevated temperature well above said low temperature for converting said material to graphite having desired microwave loss properties.

3. The method of manufacturing a microwave attenuator and support for the helix of a travelling wave tube, comprising the steps of: impregnating part of a porous ceramic rod with a sugar solution, drying the ceramic rod for evaporating the solvent of said solution, heating the rod in an inert atmosphere at a low temperature for a period of time sufiicient to partially decompose the sugar impregnated in said rod and produce a precipitate of col- Ioidal carbon in the pores of said rod, and firing the ceramic rod in an oxygen free atmosphere at an elevated temperature well above said low temperature for converting said colloidal carbon to graphite having desired microwave loss properties.

4. The method of manufacturing a microwave attenuator and support for the helix of a travelling wave tube, comprising the steps of: impregnating part of a porous ceramic rod with a sugar solution, drying the ceramic rod in air for evaporating the solvent from said solution, heatmg the rod in an inert atmosphere at a low temperature below the order of 400 C. for a period of time sufiicient to partially decompose the sugar impregnated in said rod and produce a precipitate of colloidal carbon in the pores of said rod, and firing said rod in an oxygen free atmosphere at an elevated temperature of the order of 1000 C. for converting said colloidal carbon to graphite having desired microwave loss properties.

5. The method of producing a structure suitable for absorbing microwave energy, comprising the steps of: impregnating a porous ceramic body with a solution of an organic compound, heating said body at a low temperature for gradual decomposition of said organic compound into a carbon containing material, and firing said body at an elevated temperature well above said low temperature for converting said material to graphite having desired microwave loss properties.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF MANUFACTURING A MICROWAVE ATTENUATOR AND SUPPORT FOR THE HELIX OF A TRAVELLING WAVE TUBE, COMPRISING THE STEPS OF: IMPREGNATING PART OF A POROUS CERAMIC ROD WITH A SOLUTION OF AN ORGANIC COMPOUND, HEATING SAID ROD AT A LOW TEMPERATURE FOR GRADUAL DECOMPOSITION OF SAID ORGANIC COMPOUND INTO A CARBON CONTAINING MATERIAL, AND FIRING SAID ROD AT AN ELEVATED TEMPERATURE WELL ABOVE SAID LOW TEMPERATURE FOR CONVERTING SAID MATERIAL TO GRAPHITE HAVING DESIRED MICROWAVE LOSS PROPERTIES. 